The invention relates to a marine riser joint and in particular to a marine riser joint for measuring loads exerted on a marine riser.
Drilling boreholes in underground earth formations situated below a body of water may be performed by mounting drilling equipment on a floating vessel, and arranging a marine riser between the vessel and a wellhead situated at a level near the sea bottom (or ocean bottom). Such a marine riser consists of a plurality of sections that are interconnected in end-to-end relationship. The sections consist of tubes having a relatively large diameter so that a string of interconnected sections forms a tubular communication between the wellhead and the floating vessel. Tubular equipment (such as a drill string) may be guided from the vessel into the well through this communication, which further forms a return passage for the drilling fluid that has been used in the well.
The lower end of the marine riser is connected to the wellhead. The upper end of the riser is connected to the floating vessel and loaded with an upwardly-directed force to prevent buckling of the riser. A reduction in the magnitude of the required upward force can be obtained by arranging buoyancy members around the sections.
It will be appreciated that the marine riser should preferably extend almost vertically through the water. To achieve this, the vessel is anchored (either by means of sea-anchors or dynamically by the action of propulsion means) to maintain a position that is (as nearly as possible) straight above the well. However, it will be appreciated that conditions may arise, wherein excessive wave or current action on the vessel displaces the vessel in a horizontal direction to an extent which may cause overloading of the marine riser.
To prevent damage of the marine riser, the drilling operator should be continuously informed on the load condition of the marine riser to enable him to take the required steps for counteracting excessive displacement of the floating vessel, or to take any other measures to obviate excessive loads on the marine riser.
Some indication of the load exerted on a marine riser may be obtained by calculating and/or measuring the curvature thereof. Information on the curvature allows the operator (in combination with additional data, such as the upward forces exerted by the buoyancy means, the weight of the marine riser, the lifting force exerted on the marine riser at the upper end thereof) to calculate the stresses in the marine riser at the location(s) where extreme load conditions exist.